1. Field of the Invention
The present invention relates to a high pressure control valve (expansion valve) which can be applied to a refrigerating cycle using a refrigerant, such as carbon dioxide (CO2), which is in a supercritical state.
2. Description of the Related Art
In general, in a case where CO2 is used as a refrigerant, the theoretical efficiency of the refrigerating cycle is lower than that of the HFC 134a refrigerant which has been conventionally used.
Therefore, as shown in FIG. 1, it is necessary to enhance the COP (coefficient of performance) of the refrigerating cycle by exchanging heat between the refrigerant leaving the gas cooler (radiator) 2 and the refrigerant entering the compressor 1 using an internal heat exchanger 8. When the internal heat exchanger 8 is used, the refrigerant entering the compressor is heated. Therefore, the enthalpy “i” is increased, and the refrigerant is superheated.
FIG. 8A is a graph showing an effect of the enhancement of COP in the case where the refrigerant entering the compressor is superheated in the internal heat exchanger 8. In this connection, TS in the drawing represents the refrigerant evaporation temperature of the evaporator 4. The higher the temperature of the refrigerant in the evaporator 4 is, the more the effect of the enhancement of COP is enhanced. In the case of an air conditioner for vehicle use, at the time of an idling operation, the rotating speed of the compressor 1 is lowered. Therefore, concerning the air-conditioner for vehicle use, its cooling capacity is low. As the refrigerant evaporation temperature in the evaporator 4 is raised, an effect of enhancement of COP of the internal heat exchanger 8 is increased. Accordingly, a great advantage can be provided by using the internal heat exchanger 8.
FIG. 8B is a graph showing a pressure control for controlling the pressure at which COP is maximized with respect to the temperature of the refrigerant leaving the radiator 2. As shown in the graph, the following characteristic is known. In the case where the refrigerant entering the compressor 1 is heated with the internal heat exchanger 8, the refrigerant evaporation temperature in the evaporator 4 is high. The higher the temperature of the refrigerant leaving the radiator 2, the lower the control pressure in the case where the refrigerant is superheated. In this connection, SH as shown in the drawing represents superheating.
The reason why the above characteristic is provided will be described as follows. In the Mollier chart shown in FIG. 9, in which the physical property of CO2 is shown, the refrigerant, which has been sucked by the compressor 1, ideally follows an isentropic curve and is compressed to a refrigerant at high temperature and high pressure. According to the physical property of the refrigerant of CO2, an inclination of the isentropic curve “s” is reduced on the right side of the Mollier chart where enthalpy is increased. When a comparison is made at the same pressure, as compared with a case in which a saturated gas refrigerant is sucked and compressed, an increase in the enthalpy “i” (=power of the compressor) in the case of compressing the refrigerant to the same pressure becomes larger than when the superheated refrigerant is compressed.
Therefore, in the refrigerating cycle in which CO2 refrigerant is used, a control method is known in which the pressure of the refrigerant is controlled to a high pressure at which COP is maximized with respect to the refrigerant temperature at the outlet of the radiator 2. However, in the case where the internal heat exchanger 8 is provided, as the power for driving the compressor 1 is increased, the pressure at which COP is maximized becomes low. When the control pressure is reduced as described above, an advantage can be provided in that the durability of the other high pressure parts, such as a compressor 1 and a radiator 2, can be enhanced.
At the time of idling operation of a vehicle, no air flow is generated. Accordingly, the air flow to the radiator 2 is decreased. In addition to that, due to a flow of hot air flowing from an engine compartment, a suction air temperature is raised and a temperature of the refrigerant leaving the radiator 2 is increased. Therefore, in the case where the internal heat exchanger 8 is used, it is necessary to use a high pressure control valve 3 having a control characteristic in which the control pressure is low with respect to the same temperature of the refrigerant leaving the radiator.
Concerning the high pressure control valve (expansion valve) for controlling the pressure of CO2 in the supercritical state, the official gazettes of JP-A-9-264622 (patent document 1) and JP-A-2000-193347 (patent document 2) disclose high pressure control valves which are well known.
In the above patent documents 1 and 2, as a temperature sensing section for operating a displacement member of the control valve, a high pressure control valve is shown in which the same CO2 refrigerant, as the refrigerant circulating in a refrigerating cycle, is charged into an air-tightly closed space. Especially, in the patent document 1, a high pressure control valve is shown in which a charging density of charging CO2 refrigerant into the air-tightly closed space is 450 kg/m3 to 950 kg/m3. However, the high pressure control valves shown in these patent documents 1 and 2 are applied to a refrigerating cycle in which an internal heat exchanger 8 is not used. That is, it is difficult for the high pressure control valves shown in these patent documents 1 and 2 to be applied to a refrigerating cycle including the internal heat exchanger 8.